1. Field of the Invention
This application relates to the field of speed-responsive engaging devices such as clutches.
2. Prior Art
It has been known in the prior art that it is desirable to have a clutch to lock the impeller and turbine elements of a hydrodynamic device together to eliminate slippage therebetween under certain conditions to improve the efficiency of the device. It is further known that it is convenient to use the speed of rotation of an element as a signal to initiate engagement and disengagement of the clutch. The engagement is critical, for example, when the lock-up clutch is used in a hydrodynamic device in combination with an automatic transmission. During an automatic ratio change, it is desirable the clutch be disengaged so that torque changes occurring during the shift will not be harsh, due to the hydrodynamic device absorbing shocks. If the clutch in the hydrodynamic device remains engaged during a shift, the shift control to provide smooth shifts is more exacting since the normal shock absorbing characteristics of the hydrodynamic device is eliminated. If the prime mover has objectionable torsional vibrations, it is equally important to utilize the shock absorbing characteristics of the hydrodynamic device at times.
One solution to this problem is illustrated in application Ser. No. 620,461, filed Oct. 7, 1975, of common assignee. This previous solution provided a lock-up clutch for a hydrodynamic device of a simple design which was operative to engage and disengage at the proper time without the use of external control mechanisms to determine the points of engagement and disengagement. Further, the structure of the previous application provided a lock-up clutch for a hydrodynamic device which automatically permits speed differentials within the device during specific operating conditions such as ratio change. The prior application, however, required welding or fixing of a sheet metal ramp structure to the external turbine surface which creates difficulty in that the turbine surface is normally not dimensionally controlled to the point that a perfectly operating clutch was possible in each manufactured structure. In addition, this prior solution had limited surface area on the friction surface due to interruption thereof by the spring utilized. Further, the structure required contact of the shoe over its entire lower surface with the cam requiring a wedgeshaped shoe involving machining of the lower surface of the shoe.